What is the purpose of `trigger` in the Optimistic UI example?

In this file, https://github.com/praveenweb/swr-graphql/blob/master/pages/optimistic-ui.js

What is the purpose of `trigger(mutation)`?

The official Optimistic UI example doesn't use that https://github.com/vercel/swr/blob/master/examples/optimistic-ui/pages/index.js

Thanks